EFFECTOR MECHANISMS 369 



ciliated cells independently of any nervous supply, and the progressive 

 intercellular excitation involved has been termed neuroid transmission. 

 Co-ordination of ciliary beat is evidenced, moreover, by the cilia in a single 

 cell, and, by microdissection, it has been shown that such co-ordination 

 depends on the integrity of the region containing the intracellular fibrils 

 below the basal granules. 



Reversal of the direction of ciliary beat has been reported for a number of 

 animals but, apart from the Protozoa, it is a rare phenomenon. Reversal of 

 direction of locomotion in ciliates and small turbellarians appears to 

 result from a change in the direction in which the cilia beat. In Para- 

 mecium ciliary reversal is obtained by treatment with KC1 and by applying 

 an electrical field. Parker has reported that cilia on the lips of the sea 

 anemone Metridium reverse their beat in the presence of food or KC1 ; 

 the metachronal wave is also reversed by the latter treatment. 



Cilia may be entirely autonomous or may be subject to control by the 

 organism. In addition to such examples of changes in direction and manner 

 of beat already instanced, the cilia of certain animals are subject to 

 inhibition. Gray notes that cases of ciliary control are chiefly characteristic 

 of locomotory cilia, such as those of ciliates, ctenophores, planarians, and 

 planktonic larvae of annelids, molluscs and polyzoans. Notable exceptions 

 are labial cilia of Metridium and tentacular cilia of polyzoans. 



The velar cilia of nudibranch larvae show alternations of rest and activity, 

 and can be arrested by stimulation. When a portion of the velum is re- 

 moved the cilia beat continuously, and nerve anaesthetics abolish the 

 periods of ciliary arrest in the intact larva. The ciliated velar cells are 

 innervated and it seems likely that ciliary arrest results from nervous 

 inhibition. 



Nervous control of ciliary beat is well marked in ctenophores. A form 

 such as Pleurobrachia moves through the water mouth foremost by co- 

 ordinated beating of the combs which strike in an aboral direction. At 

 faster rates of swimming the metachronal waves of the eight rows of plates 

 are synchronized; at slow rates co-ordination may be evident only in 

 two rows of a quadrant, or among the combs of a single row. In certain 

 ctenophores (especially cydippids) mechanical stimulation causes ciliary 

 reversal, attended by a change in the direction of the metachronal wave. 

 When a comb row is transected the combs on either side of the cut beat 

 independently. Nervous connexions to the combs have been demonstrated 

 and it now appears that the ciliary beat is regulated by a diffuse nerve 

 net. The statocyst (apical sense organ), which connects with each of the 

 paddle rows, maintains co-ordination between the rows and orientates the 

 animal with respect to gravity (56). 



Acetylcholine is reported to be concerned with ciliary activity; acetyl- 

 choline, cholinesterase and choline acetylase occur in ciliary tissue (gills 

 of Mytilus). Di-isopropyl fluorophosphonate (DFP) and eserine inhibit 

 acetylcholinesterase of ciliated cells. Ciliary movement is enchanced by 

 acetylcholine and eserine in low concentrations, and is depressed by 



